John has taught college science courses face-to-face and online since 1994 and has a doctorate in physiology.

All skeletal muscles have a resting length. When our muscles are stretched to the ideal length, it can maximize muscular contraction. This lesson explains the length-tension relationship in skeletal muscle and explores how the arrangement of myofilaments in a sarcomere can impact tension and contraction.

Muscles are Stretched at Rest

Tendons attach our skeletal muscles to our bones.

Tendons are organs that attach our skeletal muscles to our bones. When our muscles are resting, that is, not contracting, they are actually stretched to what we call a resting length by these attachments. How do we know this? Let's take a look at the frog's gastrocnemius, or calf muscle:

The calf muscle would shorten if it were removed from the body.

If you were to remove the muscle from the body of the frog, its length would shorten. Therefore, the muscle is stretched to its resting length within the body. As the muscle is stretched, so are the muscle fibers that make up the muscle organ.

As it turns out, the natural resting length of our skeletal muscles maximizes the ability of the muscle to contract when stimulated. If the resting length is shorter or longer, contraction is compromised. The effect of resting fiber length on muscular contraction is referred to as the length-tension relationship. This lesson will describe the anatomical arrangement of the muscle at rest and explain how this helps with muscular contraction.

Length-Tension Relations

Let's do an experiment using the gastrocnemius muscle of a frog to examine the relationship between resting muscle fiber length and contraction.

First, remove the gastrocnemius from the frog. Then, clamp the muscle between a fixed position and a force transducer, which is an instrument that will record how much contraction occurs when the muscle contracts. We can move the clamp to change the resting length of the muscle - in other words, how long the muscle is before it contracts. We will then record contraction after stretching the muscle 1mm each time.

Let's start with a short length at which the muscle is pretty loose. When the muscle contracts at this short resting length, we see a small amount of force development, as illustrated by the small blip on the picture below.

When the muscle is loose, only a small amount of force develops during contraction.

Now, let's stretch the muscle a little bit, so we increased its resting length by just 1mm. As you can see below, the muscle contracts with more force at this longer resting length. If we stretch the muscle once again to now 2mm beyond what it was originally, it develops even more force.

Stretching the muscle by 1mm allows for more force generation.

However, if we stretch the muscle 3mm beyond the original length, now the force developed is less. When we stretch the muscle 4mm, the muscle force development is even less.

At a certain stretch, the force generation will begin to decrease.

Our results can be graphed to illustrate the resting length on the x-axis versus the tension or force development on the y-axis. As you can see, tension development increases as we increase the resting length to a point, and then tension or force development decreases with further stretch. As it turns out, the resting length that produces maximum tension just so happens to be the resting length of the frog's muscle in the body.

The resting length of our muscles produces maximum tension.

Microscopic Anatomy

We need to look at the minute details of skeletal muscle cells in order to understand this relationship between resting fiber length and contraction. Specifically, we need to examine the contents of a sarcomere, which is the functional unit of a striated muscle. Because skeletal muscle is a type of striated muscle, every fiber is composed of these sarcomeres, each of which contains the necessary components for contraction.

The image you see above shows a relaxed sarcomere on top compared to a contracted sarcomere below. The Z-discs, sometime referred to as Z-lines, define the ends of the sarcomere. Thin filaments are composed of actin, and they're attached to the Z-discs, while the thick filaments reside in the middle of the sarcomere. As you can see in the contracted sarcomere, the filaments have slid over each other. In short (no pun intended...) the myosin molecules in the thick filament pull the thin filaments toward the middle of the sarcomere, thus shortening the sarcomere and causing contraction.

Length-Tension Relationship

Now that we've looked at the structure and function of an individual sarcomere, we can discuss how resting fiber length affects muscular contraction. In fact, we don't have to look at anything except an individual sarcomere to see how this works. By increasing the resting length of the sarcomere, we increase the distance that the filaments can slide over each other and, thus, develop tension or force development. However, if we stretch the sarcomere so there is no overlap between the filaments, the myosin cannot attach to the thin filament.

Graph showing the relationship between sarcomere length and muscular tension

The image above illustrates how much tension can be developed in a muscle if held at different resting lengths. Additionally, images are sketched to represent the relative arrangement of the thin and thick filaments at each given resting length - in other words, what the sarcomere looks like at that resting length. As you can see, maximum tension is developed when the muscle is held at a length that is similar to that which is in the human body (or for the frog, in the frog's body).

At the optimal resting length, a maximum amount of myofilament sliding can occur during contraction. If the resting length is shortened, the amount of shortening is compromised, as the sarcomere can only shorten so much. At longer resting lengths, it becomes more difficult for the thin and thick filaments to interact with one another and thus cause shortening and contraction.

Lesson Summary

In summary, our skeletal muscles are stretched to a resting length as they are attached to bones on each end by tendons. The normal resting length of the muscle in our bodies maximizes the amount of contraction that occurs in the muscle when stimulated. Contraction is maximized due to the relative arrangement of the myofilaments (that is, the thin and the thick filaments) within the sarcomere, the functional unit of striated muscle. The normal resting length of the sarcomere allows for maximum shortening and thus maximum contraction.

Learning Outcomes

At the conclusion of this lesson, you will be able to:

Define resting length and normal resting length

Identify the anatomy of a sarcomere

Describe how muscles generate force when contracted

Explain why maximal contraction is achieved at the normal resting length of muscles in the body

Summary:

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